Because it's not just another cloud-hosted environment.
Building a private/permissioned blockchains based on Hyperledger Fabric or on the Ethereum VM (using technologies like Quorum or Parity's POA-Config) is a non-trivial task. Creation, configuration and manual onboarding of new members is usually performed using low-level command line admin tools.
One possiblity - which some corporations follow - is to rely on a single cloud provider to host their complete blockchain. In this case, you might end up with a closed-down blockchain network, in which all nodes have to be run on this particular providers' infrastructure.
We believe that this works against the ingrained decentralization-aspect of blockchain technologies and that companies should be vary of this approach.
We're following a very different approach with Blockchain Builder.
The Blockchain Builder allows a GUI-led, transparent approach to blockchain network and node configuration. It provides a level of comfort comparable to single-operator cloud-hosted environments, but without posing any restriction on how individual nodes are operated. Your network always belongs to you and your members, and you can freely mix nodes which are hosted on your members' premises with nodes hosted at local cloud providers', and also ones running on Azure, AWS, IBM, and others.
In all cases, you retain 100% authority of operation and configuration of your network. You can cease to use Blockchain Builder and your network remains fully operational, scalable and usable. You'd just have to go back to the low-level tooling built into the chosen technology.
We curently plan to ship Blockchain Builder with support for Parity POA and Hyperledger Fabric later in 2018.
Blockchain Access Gateway
We believe in a world in which a corporation will not just interact with a single blockchain, but with dozens of them, similarly to the way you interact with commercial systems today.
To allow application developers to write code which bidirectionally integrates their existing applications with smart contracts on these different chains, we're creating Blockchain Access Gateway, a unified interface into different blockchain technologies. Our goal is to allow developers to work with code running on public or private Ethereum-based networks (including Quorum, Parity's POA config and Ethermint), Hyperledger Fabric, and Tendermint-based chains without knowing the particular details of - and differences between - these technologies.
We believe that Smart Contracts and ChainCode will be developed in different programming languages, but the way for an application developer to interact with them should be consistent and uniform.
We're planning to release Blockchain Access Gateway with support for REST and AMQP interfaces to blockchain contracts later in 2018.